Caprolactam is an important monomer for making polyamide-6 fiber, plastics and thin film. The production of caprolactam in China is not sufficient to meet the production requirement of the domestic market. Up to 2010, the total apparent consumption of caprolactam in China has exceeded 1.1 million tons, but the self-sufficiency rate is only 43.9%. At present, about 90% of the total production of caprolactam in the world adopts a liquid-phase Beckmann rearrangement process of cyclohexanone oxime using concentrated sulfuric acid or fuming sulfuric acid as a catalyst. Although this route has a selectivity of 98% or more, there are many problems such as heavy environmental pollution, serious equipment corrosion, generation of a large amount of byproduct ammonium sulphate with low value.
At present, there are many methods under research for preparing caprolactam without the byproduct of ammonium sulphate. At present, they are mainly gas-phase Beckmann rearrangement and liquid-phase rearrangement processes using a solid catalyst. But these processes usually have many problems such as the preparation cost of the solid catalyst being too high, the selectivity of the caprolactam being too low, or the catalyst being prone to inactivation and the effectiveness of reuse being too poor, thereby greatly limiting its industrialized application.
A microreactor is a three-dimensional structural element which can be used in chemical reaction and made of a solid substrate by means of a special microfabrication technology. The microreactor generally has a small channel size (its equivalent diameter being smaller than 500 μm) and a channel diversity, a fluid flows in these channels, and requires a desired reaction to take place in these channels. This causes that a chemical equipment with microstructure has a very large specific surface area to volume ratio, resulting in an extremely high mass transfer and heat transfer capacity. Fundamental advantages due to this is an extremely high heat exchange efficiency and mixing efficiency, and that the reaction temperature can be precisely controlled and the reaction materials can be instantaneously mixed in a precise mix proportion, they are key factors for increasing yield, selectivity, safety and improving product quality. At present, the application of the microreactor in the technical field of chemical synthesis is still in a new research stage, and the research of the application of the microreactor in studying the preparation of caprolactam by Beckmann rearrangement is also still in laboratory study stage. Luo Guangsheng, Zhang Jisong et al. of Tsinghua University prepared caprolactam in a microreactor by using fuming sulfuric acid or organic acid as the catalyst, its process can rapidly and efficiently synthesize caprolactam within a extremely short time, the conversion is up to 99.9% or more, and the selectivity is also greater than 99%. This demonstrates that the application of the microreactor in preparation of caprolactam has a great application prospect. But, these reactions usually require high temperature and high pressure and catalysis by a strong acid medium, thereby limiting their industrialized application to a certain extent or increasing the cost of industrialized application. Therefore, searching for methods for preparing caprolactam under a relatively mild condition and their application in industrial production is also hot subjects of current research.